10.2 Weight-Based & BSA Dosing

Weight-Based (mg/kg) Dosing

The core equation is simple: dose (mg) = prescribed dose (mg/kg) x body weight (kg). The marks are lost on the details: whether the figure is per dose or per day, how the daily dose is divided, and whether a maximum cap applies.

Worked Example 1: Pediatric Amoxicillin, Divided Daily Dose

Question: Amoxicillin 40 mg/kg/day in three divided doses (q8h) for a child weighing 20 kg. What is each dose?

1. Total daily dose: 40 mg/kg/day x 20 kg = 800 mg/day.
2. Divide by the number of doses: 800 / 3 = 266.7 mg per dose.
3. Round to a practical, measurable dose: 800 mg/day in three divided doses is conventionally given as about 250 mg per dose (the 250 mg/5 mL suspension allows an easy 5 mL measure, and the small shortfall is clinically acceptable).

Worked Example 2: Single-Dose Gentamicin

Question: Once-daily gentamicin 5 mg/kg IV for a 70 kg adult. The vial contains 40 mg/mL. What dose and volume?

1. Dose: 5 mg/kg x 70 kg = 350 mg.
2. Volume: 350 mg / 40 mg/mL = 8.75 mL.

Note the difference from Example 1: here the order is per dose (given once daily), so no division step is needed. Reading "mg/kg/day" as "mg/kg per dose" - or vice versa - is the most common way these questions are failed.

Maximum Daily Dose Caps

Many pediatric orders carry an adult-style ceiling. Always calculate the weight-based dose, then compare it with the maximum and give the lower value.

Worked Example 3: Applying the Cap

Question: A drug is dosed at 50 mg/kg/day, maximum 2 g/day.

• Child A, 30 kg: 50 x 30 = 1500 mg/day. Below the cap, so give 1500 mg/day.
• Child B, 50 kg: 50 x 50 = 2500 mg/day. This exceeds the cap, so give 2000 mg/day - not 2.5 g.

Exam distractors typically include the uncapped figure (2500 mg), so write both numbers down and circle the lower one.

Ideal Body Weight and Adjusted (Dosing) Weight

Hydrophilic drugs such as the aminoglycosides distribute poorly into adipose tissue. Dosing an obese patient on actual body weight overdoses them; dosing on ideal body weight underdoses them. The standard approach:

1. Ideal body weight (Devine formula):
   • Male: IBW = 50 kg + 2.3 kg for each inch over 5 feet
   • Female: IBW = 45.5 kg + 2.3 kg for each inch over 5 feet
2. If actual body weight is within roughly 120-130% of IBW, dose on actual weight.
3. If the patient is obese (actual weight well above IBW), use adjusted (dosing) body weight = IBW + 0.4 x (actual weight - IBW).

Worked Example 4: Gentamicin in an Obese Patient

Question: A man is 5 ft 7 in (67 inches) tall and weighs 100 kg. Calculate his gentamicin 5 mg/kg dose using adjusted body weight.

1. IBW = 50 + (2.3 x 7) = 50 + 16.1 = 66.1 kg.
2. Actual weight is 100 / 66.1 = 151% of IBW, so he is obese for dosing purposes.
3. Adjusted weight = 66.1 + 0.4 x (100 - 66.1) = 66.1 + 0.4 x 33.9 = 66.1 + 13.56 = 79.7 kg, about 80 kg.
4. Dose = 5 mg/kg x 79.7 kg = 398 mg, practically 400 mg.

Dosing on actual weight (500 mg) or ideal weight (330 mg) are the two classic wrong answers.

Body Surface Area Dosing: The Mosteller Formula

Body surface area (BSA) correlates better with metabolic rate and organ function than weight alone, so chemotherapy and some pediatric drugs are dosed in mg/m2. The exam-standard formula is the Mosteller formula:

BSA (m2) = square root of [ height (cm) x weight (kg) / 3600 ]

Worked Example 5: Calculating BSA

Question: A patient is 170 cm tall and weighs 70 kg. Calculate the BSA.

1. Multiply height by weight: 170 x 70 = 11,900.
2. Divide by 3600: 11,900 / 3600 = 3.306.
3. Take the square root: square root of 3.306 = 1.818, so BSA about 1.82 m2.

An average adult BSA of roughly 1.7-1.9 m2 is a useful sanity check; a calculated BSA of 3 m2 or 0.5 m2 for an adult signals an arithmetic error.

Worked Example 6: mg/m2 Dosing

Question: A chemotherapy drug is dosed at 40 mg/m2 for the patient above (BSA 1.82 m2).

1. Dose = 40 mg/m2 x 1.82 m2 = 72.8 mg.
2. Round to a practical dose given vial sizes, for example 73 mg or 70 mg per local protocol (many oncology protocols permit rounding within about 5% to avoid wastage).

Historical Rules: Clark's and Young's

These appear occasionally as distractors or in "which method" questions. Both scale the adult dose down; modern practice prefers mg/kg or BSA, but you should recognise them.

• Clark's rule: child's dose = (weight in lb / 150) x adult dose. A 75 lb child with a 500 mg adult dose: (75 / 150) x 500 = 250 mg.
• Young's rule: child's dose = [ age / (age + 12) ] x adult dose. A 6-year-old with a 500 mg adult dose: (6 / 18) x 500 = 0.333 x 500 = about 167 mg.

These rules ignore pharmacokinetics entirely; treat them as historical context, not a dispensing method.

Rounding Rules for Practical Dispensing

• Round oral liquid doses to a measurable volume: with an oral syringe, to the nearest 0.1 mL; with a dosing spoon, to the nearest 2.5 mL.
• Round tablet doses to the nearest whole or half tablet (only if the tablet is scored and not modified-release or enteric-coated).
• Keep the rounded dose within about plus or minus 5-10% of the calculated dose; if rounding forces a larger deviation, choose a different strength or formulation.
• For narrow-therapeutic-index drugs (digoxin, aminoglycosides, chemotherapy), document the exact calculated dose and round conservatively rather than upward.

Exam Traps

• Confusing mg/kg per dose with mg/kg per day - read the order twice.
• Forgetting the maximum daily dose cap on heavy children.
• Dosing aminoglycosides on actual weight in obesity instead of IBW + 0.4 x excess.
• Mosteller errors: forgetting the square root, or using weight in lb or height in inches - the formula needs kg and cm only.
• Applying Clark's or Young's rule when the question asks for a modern mg/kg or mg/m2 calculation.